The present invention relates to a pyrochemical reprocessing method of spent nuclear fuel used in atomic reactor and to an induction heating system used in the pyrochemical reprocessing method.
Studies have been performed, both domestic and abroad, on recycling method to improve economic feasibility of nuclear fuel cycle, by which the spent nuclear fuel is reprocessed by pyrochemical reprocessing method using molten salt and uranium or plutonium and the spent fuel is collected and recycled.
In the pyrochemical reprocessing of oxide fuel, spent nuclear fuel is dissolved in a molten salt, and oxides of uranium or plutonium in granular state are deposited and are collected by electrolysis. Principal processes are as follows:    Chlorination dissolving process of spent fuel::UO2+Cl2→UO2Cl2:PuO2+C+Cl2→PuCl4+CO2    Electrolysis and collecting process of uranium oxide (cathode)UO2Cl2→UO2+Cl2     Precipitating and collecting process of plutonium oxidePuCl4+O2→PuO2+2Cl2
The crucible used for reprocessing is made of pyro-graphite, and it exerts action as an anode in the electrolysis and depositing process.
In the pyrochemical reprocessing method for metal fuel, spent nuclear fuel is dissolved in a molten salt. Metal uranium or metal plutonium are deposited and are collected by electrolysis. Principal processes are as follows:    Dissolving processing of spent fuel:U→U3++3e−; Pu→Pu3++3e−    Electrolysis and collecting process of metal uranium (solid cathode):U3++3e−→U    Electrolysis and collecting process of uranium oxides or plutonium oxides (liquid cathode):U3++3e−→U; Pu3++3e−→Pu    (1) In the conventional heating method used for the melting of the spent nuclear fuel in the molten salt, the crucible is directly heated. As a result, the crucible acts as heat transmitting surface, and it turns to a corrosion environment at higher temperature than the melting temperature of the salt.    (2) The crucibles are directly exposed to chlorine gas, oxygen gas, etc. used in the pyrochemical reprocessing, and they make severe corrosion environment.    (3) Compared with induction current used in the conventional metal melting method, higher frequency is needed due to the difference in electric conductivity.    (4) It is generally practiced to adopt water-cooling method in order to cool down the crucible used for induction heating. However, when water is brought into contact with the molten salt by any chance, explosion may occur.    (5) In the conventional heating method, stirring means are required to evenly maintain temperature distribution of the molten salt, and this requires the designing of the entire system in more complicated structure.    (6) In the resistance heating method, the molten salt is evenly molten and evenly stirred up, and a cylindrical crucible must be generally adopted.